Need a test to check sdlvga output #77
Description
The SDL output devices are some of the most user-visible parts of the simulator, but their behavior is not rigorously tested. Some bugs (like #76) can creep in because manual testing is not frequent enough to catch errors that manifest only in some complex codes.
Add an test to continuous integration that captures a screenshot of the sdlvga output device after a configurable number of instructions executed, and compares the screenshot against a reference image.